Fluid-working machines include fluid-driven and/or fluid-driving machines, such as pumps, motors, and machines which can function as either a pump or as a motor in different operating modes.
When a fluid-working machine operates as a pump, a low pressure manifold typically acts as a net source of a working fluid and a high pressure manifold typically acts as a net sink for a working fluid. When a fluid-working machine operates as a motor, a high pressure manifold typically acts as a net source of a working fluid and a low pressure manifold typically acts as a net sink for a working fluid. Within this description and the appended claims, the terms “high pressure” and “low pressure” are relative, and depend on the particular application. In some embodiments, low pressure working fluid may be at a pressure higher than atmospheric pressure, and may be several times atmospheric pressure. However, in all cases, low pressure working fluid will be at a lower pressure than high pressure working fluid. A fluid-working machine may have more than one low pressure manifold and more than one high pressure manifold.
Large displacement ring cam fluid-working machines (i.e. fluid-working machines having a large rotating annular cam driving a plurality of radial pistons arranged around the cam, with each piston typically reciprocating multiple times per cam revolution) are known and are proposed for use in renewable energy applications in which there is a low speed rotating input but a relatively high speed electrical generator (Rampen, Taylor & Riddoch, Gearless transmissions for wind turbines, DEWEK, Bremen, December 2006). Ring cam fluid-working machines typically have a plurality of rollers rolling on a wave shaped cam and operatively connected to pistons. Each piston is slideably engaged in a cylinder, the cylinder and piston together defining a working chamber containing working fluid, in communication via one or more valves with high and low pressure manifolds. The pistons are each operable to undergo reciprocating motion within the cylinder so as to vary the working chamber volume, when the ring cam rotates, such that a cycle of working chamber volume is executed, and during which working fluid may be displaced.
Ring cam fluid-working machines may be configured so that the pistons and cylinders are located inside the ring cam, the ring cam having an inward facing working surface, or may be configured so that the ring cam has an outward facing working surface and is located inside the pistons and cylinders. Indeed, ring cam fluid-working machines of either configuration are also known in which either the ring cam rotates, or the pistons and cylinders rotate. It is also possible for the ring cam to have both inward and outward facing working surfaces where the ring cam is located between inner and outer rings of pistons and cylinders. It is even possible for the pistons and cylinders to be aligned roughly parallel with the axis of rotation, and for the ring cam to have one or more axially facing working surfaces.
Multi-cylinder fluid-working machines, including ring cam fluid-working machines, may be variable displacement fluid-working machines (either pumps or motors, or machines operable as either pumps or motors), wherein each working chamber is selectable to execute an active (or part-active) cycle of working chamber volume in which there is a net displacement of working fluid, or an idle cycle in which there is substantially no net displacement of working fluid, by the working chamber during a cycle of working chamber volume, for regulating the time-averaged net displacement of fluid from the low pressure manifold to the high pressure manifold or vice versa.
Large fluid-working machines (such as those suitable for renewable energy generation) are typically subject to particularly high internal forces and pressures. For example, the pressure of the high (and indeed low pressure) working fluid of a large scale ring cam fluid-working machine, of a size suitable for a wind turbine, is particularly high. Consequently the forces received by the ring cam from the rollers are also high, and it is known for the ring cam working surfaces to degrade. It has been proposed to assemble large scale ring cams from a number of segments, and it is known for excessive wear to occur to the roller and to the working surface due to discontinuities which appear on the working surface under pressure of a roller at the interface between segments.
In particular, when the operating pressure of ring cam fluid working machines is very high (for example, higher than 300 Bar), the repetitive surface stress (Hertzian stress) in the ring cam and roller can exceed levels (for example, 1.5 GPa) which allow a long working life for the ring cam. Additionally, it is desirable to have a high number of lobes on the ring cam (shortest wavelength) to increase the speed multiplication factor (the factor by which the working chamber cycle frequency is increased over the shaft rotation rate), but the Hertzian stress in the working surface increases with increasing slope of the ring cam surface. Thus it is not possible simply to make the rollers larger for the same size of piston, because the piston would anyway only apply force to the roller over the same area, nor to have more or higher amplitude waves, or the machine would become larger and heavier. The curvature of the cam is also important in that the curvature of the cam determines the contact area between the cam and the roller.
Accordingly, there remains a need for a fluid-working machine and a ring cam for a radial fluid-working machine of minimum weight, maximum speed multiplication factor, and having extended working lifetime.